1. Field of the Invention
The present invention relates to an electromagnetic-wave suppressing material used to suppress undesired electromagnetic radiation from an electronic apparatus, an electromagnetic-wave suppressing device including such an electromagnetic-wave suppressing material, and an electronic apparatus having the electromagnetic-wave suppressing device including the electromagnetic-wave suppressing material.
2. Description of the Related Art
An increase in use of high-frequency electromagnetic waves in recent years has newly caused environmental disadvantages such as malfunction of an apparatus due to electromagnetic wave noise and undesirable influences of such electromagnetic radiation on the human's brain and body. For example, a 2.45 GHz band, one of frequency bands available in license-exempt wireless communications, has been used in many apparatuses for wireless LAN (IEEE 802.11b), Blue Tooth®, ISM (Industrial, Scientific and Medical), and so on. Furthermore, speedup and digitalization of clock frequencies of information apparatuses may cause harmonics in such a frequency band. Thus, risks of the occurrence of interference are highly increasing because of exponential increases in numbers and diversities of both potential electromagnetic wave source and interference-damage receiving side.
In order to cope with such electromagnetic interferences (EMI), individual apparatuses may need to be sufficiently resistant to any influence of electromagnetic waves coming from the outside (i.e., an improvement in immunity). Simultaneously, the apparatuses may need to be prevented from radiating undesired electromagnetic waves which may interfere with normal operations of other apparatuses (i.e., suppression of emission). Such requirements are collectively referred to as electromagnetic compatibility (EMC). Various standards have been defined for allowing electronic apparatuses to ensure electromagnetic compatibility under electromagnetic environment.
For obtaining EMC in circuit design, a disturbance-suppressing element has been mainly used as a circuit element for preventing an electromagnetic disturbance wave from entering an electronic apparatus in addition to reducing the electromagnetic disturbance wave generated from the electronic apparatus.
Examples of the disturbance-suppressing element include a varistor and an LC filter which is a combination of capacitors and induction coils. These elements are designed so that they have a small loss when desired signals pass through the elements and have a large reflection loss and passage loss against disturbance waves. These elements can be combined by any suitable method and used in almost all electronic circuits.
However, the combination of a disturbance-suppressing element and a circuit element may cause a specific resonance frequency. In this case, voltage and current waveforms may oscillate to distort a desired signal waveform. Besides, the wavelength of an electromagnetic wave in the range of GHz band frequencies may be close to the circuit length of an electromagnetic circuit. Thus, the circuit itself may act as an antenna for the electromagnetic wave, possibly causing the circuit to malfunction.
Thus, when the EMC may not be obtained at the stage of circuit design, it has been attempted to obtain EMC in the stage of packaging design.
In recent years, an attention has been drawn to the use of a material for suppressing or absorbing an electromagnetic wave (hereinafter, collectively referred to as an “electromagnetic-wave suppressing material”) in the form of a sheet prepared by mixing magnetic powder with resin.
The principle of the electromagnetic-wave absorption in the electromagnetic-wave suppressing or absorbing material is a conversion of most electromagnetic-wave energy incident thereon into thermal energy in the inside of the material. Therefore, each of the electromagnetic-wave suppressing and absorbing materials can lower the amounts of both the energy reflected in the forward direction and the energy permeated in the backward direction.
Here, mechanisms of converting electromagnetic-wave energy to thermal energy can be mainly classified into three types: conduction loss, dielectric loss, and magnetic loss. Electromagnetic-wave absorption energy per unit volume, P [W/m3], is expressed by the following Equation 1 using electric field E, magnetic field H, and electromagnetic-wave frequency f.
                    P        =                                            1              2                        ⁢            σ            ⁢                                                          E                                            2                                +                      π            ⁢                                                  ⁢            f            ⁢                                                  ⁢                          ɛ              ″                        ⁢                                                          E                                            2                                +                      π            ⁢                                                  ⁢            f            ⁢                                                  ⁢                          μ              ″                        ⁢                                                          H                                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            Electric conductivity: σComplex permittivity: ∈=∈′−j∈″Complex magnetic permeability: μ=μ′−jμ″
In the Equation 1, the first term represents conduction loss, the second term represents dielectric loss, and the third term represents magnetic loss.
Magnetic materials have been typically used as materials for suppressing electromagnetic waves in the high-frequency band.
Thus, magnetic sheets made of magnetic materials are designed to increase the magnetic permeability, the third term μ″ of the above Equation 1, for suppressing and absorbing electromagnetic waves.
On the other hand, the present inventors have proposed a material having a high permittivity ∈″, the second term of the above Equation 1, indicating the dielectric loss at a frequency in the MHz or GHz band (see Japanese Unexamined Patent Application Publication No. 2006-73991).
The present inventors have paid their attention to the permittivity of a liquid material with ions such as an electrolyte and have proposed an electromagnetic-wave suppressing material with a high electromagnetic-wave absorbing efficiency as described above. The electrolyte, which contains ions, causes ionic conduction in response to the applied electric field. Unless it is a superconductive material, there is a resistance component in the electrolyte. Accordingly, the ionic conductance is influenced by the amount of such resistance component, though depending on the kind of a solvent or the like. Thus, such a resistance component may correspond to the loss part ∈r″ of the specific permittivity. In addition, the loss part ∈r″ of the specific permittivity may have a value in the range of several tens to several hundreds or more at a frequency of 1 GHz or less. In other words, the electrolyte with ions may convert the incident electromagnetic-wave energy into Joule heat and also may absorb the energy.
However, any water-containing material, such as an electrolyte, may require a difficult technology in terms of volatilization of water while securing the reliability thereof in “property retention for several years or for ten or more years”.
Therefore, the feature of preventing water from volatilization may need to be added not to the electromagnetic-wave suppressing material but a material laminated thereon.
In consideration of the above reliability, the present inventors have proposed an electromagnetic-wave suppressing material using an ionic liquid (ion liquid) containing only ions (see Japanese Unexamined Patent Application Publication No. 2007-27470).
The use of the ionic liquid (ion liquid) containing only ions leads to an increase in the amount of electromagnetic-wave suppression/absorption. Further, volatilization of a liquid material can be prevented by taking advantage of its boiling point of several hundred degrees Celsius.
Furthermore, the ionic liquid containing only ions has the properties of nonvolatility, nonflammability, thermal stability, chemical stability, high ionic conductivity, and electric polarization tolerance.